1. Field of the Invention
Methods and apparatuses consistent with the present invention relate to a handoff for mobile nodes, and more particularly to a handoff in FMIPv6 for seamless TCP packet transmissions.
2. Description of the Related Art
Recently, more people use wireless internet due to the rapid spread of the internet, advancement of wireless communication technologies, and improved performance of mobile terminals such as portable computers, Personal Digital Assistants (PDAs), etc. Under such wireless internet environments, the mobile terminals change their points for connections to networks frequently as they roam around. Such mobile terminals are referred to as mobile nodes.
In order to enable the mobile nodes to access the wireless internet, the high-quality internet service has to be served to the mobile nodes to the same extent as in a home network even though the mobile nodes may move to a foreign network out of their home network. Diverse technologies have been proposed for stable wireless internet services even when the mobile nodes change points for connections to networks. In particular, the Mobile Internet Protocol (IP) Working Group of the Internet Engineering Task force (IETF) has proposed a method for all mobile terminals to continuously use a specific identifier of IP address regardless of network connection points, and continues to define protocols for mobile IPs. Further, in order to solve a problem of the existing IP version 4 (IPv4) address system such as insufficient addresses for meeting increasing address demands, the Mobile IPv6 is being introduced to provide wireless internet services using the IPv6. The mobile IP version 6 (IPv6) is revised up to IETF Internet-Draft version 24 from the original proposal thereof, and may be turned into the Request For Comments (RFC) sooner or later.
The mobile IPv6 creates a new Care-of Address (CoA) upon handoff, and has inevitable delay factors such as movement detection, IP address configuration, and location update, until the completion of the newly-created address registration. The total delay caused by the delay factors can be large enough to be avoided when real-time applications or applications sensitive to losses are involved. As a technology for reducing such delays, the Fast Mobile IPv6 (FMIPv6) has been proposed which can immediately deliver data when a new link is detected as well as deliver packets to mobile terminals immediately at the time of connection to a new link.
However, even in such FMIPv6 environments, if the mobile nodes are not physically connected to any wireless access point during the handoff delay time period, the mobile nodes can not receive Transmission Control Protocol (TCP) packets sent from a correspondent node (CN) to communicate therewith, nor send an acknowledgement to the correspondent node CN. That is, the FMIPv6 environment causes communication cutoff events due to characteristics of the TCP during TCP-based communications of the mobile nodes, as the mobile nodes roam around, two occasions of which can be considered and described in detail below.
First, description will be made for the event where the handoff ends before the TCP retransmission timeout (RTO) is issued. In this case, during the handoff time period, the TCP window of the correspondent node CN does not slide since the correspondent node CN can not receive a TCP acknowledgement (ACK) from a mobile node, and, if the TCP ACK is sent through a new access router NAR after the mobile node completes its handoff, the window of the correspondent node CN slides again. That is, the packet transmission rate decreases only while the window of the correspondent node CN stops its sliding, and, after the handoff is completed, the packet transmissions resume with a window size previously used.
Next, description will be made for the event where the TCP timeout is caused due to a longer handoff time period during the handoff performance. In this case, since the correspondent node CN can not receive the TCP ACK until the TCP timeout occurs before the handoff completion, the window of the correspondent node CN does not slide, and reduces its size to 1 due to the TCP slow-start. If the window size is reduced as above, it takes much time to restore the window size up to the previous window size even when the TCP ACK is sent to the correspondent node CN through the new access router, after the mobile node completes the handoff. Further, since the correspondent node CN can send only as many packets as corresponds to the window size and due to the limitation for the number of packets that can be sent until the window size of the correspondent node CN is restored up to the previous window size, the packet transmission rate becomes lower.